1. Field of the Invention
The present invention relates to a method and a device for determining the air bleed on an aircraft turbojet engine.
2. Description of the Related Art
It is known that an aircraft such as, for example, an airplane, comprises a fuselage in which are arranged a cockpit and a passenger cabin and at least one power plant, notably formed of turbojet engines. Air bleeds are carried out by a system, referred to as an air bleed system, comprising a plurality of valves and a precooler. These valves allow the bleeding of air in various air flows flowing in or around the turbojet engine, for example a high pressure air flow, at high temperature, bled at the level of the engine, an intermediate pressure air flow, at lower temperature, also bled at the level of the engine and a cold fan air flow, bled at the level of the fan. The high pressure air flow or the intermediate pressure air flow can be supplied to the precooler. The precooler then allows a heat exchange between the flow (high pressure or intermediate pressure) thus received and the fan air flow making it possible to lower the temperature of said flow (high pressure or intermediate pressure) at the outlet of the precooler in order to supply, for example to the cockpit or to the passenger cabin of the aircraft, a flow at a regulated temperature. Such regulation is obtained by controlling the opening of a valve for bleeding the air flow.
In order to control an engine, the usual logics used for this purpose must know, in real time, the air bleed levels (aircraft and engine on the compressor and the HP (high pressure) or IP (low pressure) bleed port.
It the aircraft data on the bled air flows are too inaccurate or unreliable (the valid error rate of the aircraft data is too high) with respect to feared events relating to the engine (loss of thrust), the engine control system can be made to include a flow measurement on the engine (via a venturi or a diaphragm or across the exchanger of the air bleed system).
It is furthermore known that, for the purpose of ensuring the same thrust of the engine (same engine speed, same pressure in the chamber) in the case of an air bleed by the aircraft, the engine control loop will increase the fuel injection rate. Protections known as anti-surge or C/P means are generally provided by means of an open loop stop limiting the maximum or minimum injectable fuel flow at a given time of the acceleration or of the deceleration.
It is always necessary to have an exact and reliable estimation of the air bleed. In fact, in general it is known that:
an under-estimation of the bleed flow can result in a backing-off. The under-estimation of the flow, referred to as WBAS, results in a C/P stop very close to the C/P operating point. As the engine needs to increase the injection of fuel in order to maintain the same engine speed, there is a risk of saturating the metering system very quickly and therefore of having a backing-off of the engine resulting in a loss of thrust which can go as far as shutting down the engine; and
an over-estimation of the bleed flow results in a loss of the surge protection. The over-estimation of the WBAS flow results in a C/P acceleration stop that is higher with respect to the variation of the C/P operating line and of the surge line. This risk of surge related to the loss of surge protection can result in an undesirable loss of thrust (able to go as far as shutting down the engine).
For reasons of reliability, there is generally available, in a computer which is used for determining the air bleed, two separate channels for acquiring and calculating the bleed air flow. More precisely, it is known that one of said channels uses, for this purpose, a PBAS pressure measurement taken by a pressure sensor provided at the outlet of the precooler. It is known that, if it is desired to fly the aircraft and supply it with air, it must be possible to correct the C/P air bleeds. Thus, it is necessary to have available the PBAS measurement which contributes to the calculation of the density of the air in the calculation of the flow bled from the engine, as explained below.
The aircraft must therefore have two operational calculation channels available.